Aluminum-containing films derived from using hydrogen and oxygen gas in sputter deposition

ABSTRACT

An aluminum-containing film having an oxygen content within the film. The aluminum-containing film is formed by introducing hydrogen gas and oxygen gas along with argon gas into a sputter deposition vacuum chamber during the sputter deposition of aluminum or aluminum alloys onto a semiconductor substrate. The alumininum-containing film so formed is hillock-free and has low resistivity, relatively low roughness compared to pure aluminum, good mechanical strength, and low residual stress.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method of sputter deposition of analuminum-containing film onto a semiconductor substrate, such as asilicon wafer. More particularly, the invention relates to usinghydrogen and oxygen gas with argon during the deposition of aluminum oraluminum alloys to form an aluminum-containing film which is resistantto hillock formation.

2. State of the Art

Thin film structures are becoming prominent in the circuitry componentsused in integrated circuits ("ICs") and in active matrix liquid crystaldisplays ("AMLCDs"). In many applications utilizing thin filmstructures, low resistivity of metal lines (gate lines and data lines)within those structures is important for high performance. For examplewith AMLCDs, low resistivity metal lines minimize RC delay which resultsin faster screen refresh rates. Refractory metals, such as chromium(Cr), molybdenum Mo), tantalum (Ta), and tungsten (W), have resistanceswhich are too high for use in high performance AMLCDs or ICs.Additionally, the cost of refractory metals is greater thannon-refractory metals. From the standpoint of low resistance and cost,aluminum (Al) is a desirable metal. Furthermore, aluminum isadvantageous because it forms an oxidized film on its outer surfaceswhich protects the aluminum from environmental attack, and aluminum hasgood adhesion to silicon and silicon compounds.

An aluminum film is usually applied to a semiconductor substrate usingsputter deposition. Sputter deposition is generally performed inside thevacuum chamber where a solid slab (called the "target") of the desiredfilm material, such as aluminum, is mounted and a substrate is located.Argon gas is introduced into the vacuum chamber and an electrical fieldis applied between the target and the substrate which strikes a plasma.In the plasma, gases are ionized and accelerated, according to theircharge and the applied electrical field, toward the target. As the argonatoms accelerate toward the target, they gain sufficient momentum toknock off or "sputter" atoms and/or molecules from the target's surfaceupon impact with the target. After sputtering the atoms and/or moleculesfrom the target, the argon ions, the sputtered atoms/molecules, argonatoms and electrons generated by the sputtering process, form a plasmaregion in front of the target before coming to rest on the semiconductorsubstrate, which is usually positioned below or parallel to the targetwithin the vacuum chamber. However, the sputtered atoms and/or moleculesmay scatter within the vacuum chamber without contributing to theestablishment of the plasma region and thus not deposit on thesemiconductor substrate. This problem is at least partly resolved with a"magnetron sputtering system" which utilizes magnets behind and aroundthe target. These magnets help confine the sputtered material in theplasma region. The magnetron sputtering system also has the advantage ofneeding lower pressures in the vacuum chamber than other sputteringsystems. Lower pressure within the vacuum chamber contributes to acleaner deposited film. The magnetron sputtering system also results ina lower target temperature, which is conducive to sputtering of low melttemperature materials, such as aluminum and aluminum alloys.

Although aluminum films have great advantages for use in thin filmstructures, aluminum has an unfortunate tendency to form defects, called"hillocks". Hillocks are projections that erupt in response to a stateof compressive stress in a metal film and consequently protrude from themetal film surface.

There are two reasons why hillocks are an especially severe problem inaluminum thin films. First, the coefficient of thermal expansion ofaluminum (approximately 23.5×10⁻⁶ /° C.) is almost ten times as large asthat of a typical silicon semiconductor substrate (approximately2.5×10⁻⁶ /° C.). When the semiconductor substrate is heated duringdifferent stages of processing of a semiconductor device, the thinaluminum film, which is strongly adhered to the semiconductor substrate,attempts to expand more than is allowed by the expansion of thesemiconductor substrate. The inability of the aluminum film to expandresults in the formation of the hillocks to relieve the expansionstresses. The second factor involves the low melting point of aluminum(approximately 660° C.), and the consequent high rate of vacancydiffusion in aluminum films. Hillock growth takes place as a result of avacancy-diffusion mechanism. Vacancy diffusion occurs as a result of thevacancy-concentration gradient arising from the expansion stresses.Additionally, the rate of diffusion of the aluminum increases veryrapidly with increasing temperature. Thus, hillock growth can thus bedescribed as a mechanism that relieves the compressive stress in thealuminum film through the process of vacancy diffusion away from thehillock site, both through the aluminum grains and along grainboundaries. This mechanism often drives up resistance and may cause opencircuits.

The most significant hillock-related problem in thin film structuremanufacturing occurs in multilevel thin film structures. In suchstructures, hillocks cause interlevel shorting when they penetrate orpunch through a dielectric layer separating overlying metal lines. Thisinterlevel shorting can result in a failure of the IC or the AMLCD. Sucha shorted structure is illustrated in FIG. 11.

FIG. 11 illustrates a hillock 202 in a thin film structure 200. The thinfilm structure 200 comprises a semiconductor substrate 204, such as asilicon wafer, with a patterned aluminum layer 206 thereon. A lowerdielectric layer 208, such as a layer of silicon dioxide or siliconnitride, is deposited over the semiconductor substrate 204 and thepatterned aluminum layer 206. The lower dielectric layer 208 acts as aninsulative layer between the patterned aluminum layer 206 and an activelayer 210 deposited over the lower dielectric layer 208. A metal line212 is patterned on the active layer 210 and an upper dielectric layer214 is deposited over the metal line 212 and the active layer 210. Thehillock 202 is shown penetrating through the lower dielectric layer 208and the active layer 210 to short with the metal line 212.

Numerous techniques have been tried to alleviate the problem of hillockformation, including: adding elements, such as tantalum, cobalt, nickel,or the like, that have a limited solubility in aluminum (however, thisgenerally only reduces but not eliminates hillock formation); depositinga layer of tungsten or titanium on top or below the aluminum film(however, this requires additional processing steps); layering thealuminum films with one or more titanium layers (however, this increasesthe resistivity of the film); and using hillock resistant refractorymetal films such as tungsten or molybdenum, rather than aluminum(however, as previously mentioned, these refractory metals are not costeffective and have excessive resistivities for use in high performanceICs and AMLCDs).

In particular with AMLCDs, and more particularly with thin filmtransistor-liquid crystal displays ("TFT-LCDs"), consumer demand isrequiring larger screens, higher resolution, and higher constrast. AsTFT-LCDs are developed in response to these consumer demands, the needfor metal lines which have low resistivity and high resistance tohillock formation becomes critical.

Therefore, it would be advantageous to develop an aluminum-containingmaterial which is resistant to the formation of hillocks and a techniquefor forming and an aluminum-containing film on a semiconductor substratewhich is substantially free from hillocks, while using inexpensive,commercially-available, widely-practiced semiconductor devicefabrication techniques and apparatus without requiring complexprocessing steps.

SUMMARY OF THE INVENTION

The present invention relates to a method of introducing hydrogen andoxygen gas along with argon gas into a sputter deposition vacuum chamberduring the sputter deposition of aluminum or aluminum alloys onto asemiconductor substrate, including but not limited to glass, quartz,aluminum oxide, silicon, oxides, plastics, or the like, and to thealuminum-containing films resulting therefrom.

The method of the present invention involves using a standard sputterdeposition chamber, preferably a magnetron sputter deposition chamber,at a power level of between about 1 and 4 kilowatts (KW) of directcurrent power applied between a cathode (in this case the aluminumtarget) and an anode (flat panel display substrate--i.e., soda limeglass) to create the plasma (after vacuum evacuation of the chamber).The chamber is maintained at a pressure of between about 0.5 and 2.5millitorr with an appropriate amount of argon gas, hydrogen gas, andoxygen gas flowing into the chamber. The argon gas is preferably fed ata rate between about 25 and 90 standard cubic centimeters per minute("sccm"). The hydrogen gas is preferably fed at a rate between about 50and 400 sccm. The oxygen gas is preferably fed at a rate between about0.25 and 2 sccm (preferably in an atmospheric air stream). The ratio ofargon gas to hydrogen gas is preferably between about 1:1 and to about1:6. The films with higher hydrogen/argon ratios exhibited smoothertexture than lower hydrogen/argon ratios. The deposition process isconducted at room temperature (i.e., about 22° C.).

The aluminum-containing films resulting from this method have an averageoxygen content between about 12 and 30% (atomic) oxygen in the form ofaluminum oxide (Al₂ O₃) with the remainder being aluminum. Thealuminum-containing films exhibit golden-yellow color when formed underthe process parameters described. The most compelling attribute of thealuminum-containing films resulting from this method is that they arehillock-free, even after being subjected to thermal stresses.

Although the precise mechanical and/or chemical mechanism for formingthese aluminum-containing films is not completely understood, it appearsthat the hydrogen gas functions in the manner of a catalyst fordelivering oxygen into the aluminum-containing films. Although the flowof the oxygen gas into the vacuum chamber is small compared to the flowof argon gas and hydrogen gas, there is a relatively large percentage ofoxygen present in the deposited aluminum-containing films. Inexperiments by the inventors, oxygen gas was introduced into the vacuumchamber, without any hydrogen gas being introduced (i.e., only oxygengas and argon gas introduced). The resulting films deposited on thesubstrate did not have a measurable amount (by x-ray photoelectronspectroscopy) of oxygen present.

As stated previously, oxygen is present in the depositedaluminum-containing film in the form of aluminum oxide. However,aluminum oxide is an insulator. It is counter-intuitive to form aninsulative compound (which should increase the resistivity of the film)in a film which requires very low resistivity. However, it has beenfound that the formation of the aluminum oxide does not interrupt theconducting matrix of aluminum grains within the aluminum-containingfilm. Thus, the resistivity of the aluminum-containing film issurprisingly low, in the order of between about 6 and 10 micro ohm-cm.This is particularly striking in light of the fact that aluminum oxideis present in the range of between about 12 and 30% (atomic). The grainsize of these aluminum-containing films is between about 400 and 600angstroms (Å).

Aside from being substantially hillock-free and having a low resistivity(i.e., high conductivity), the resultant aluminum-containing films haveadditional desirable properties including low roughness, low residualstress, and good mechanical strength (as determined by a simple scratchtest compared to pure aluminum or by the low compressive stress (betweenabout -5×10⁸ and -1×10⁹ dyne/cm²), which is considered to be anindication of high scratch resistance). Measurements of thealuminum-containing films have shown that the roughness before and afterannealing is low compared to pure aluminum (about 600-1000 Å beforeannealing and 400-550 Å after annealing). Low roughness prevents stressmigration, prevents stress-induced voids, and, consequently, preventshillock formation. Additionally, low roughness allows for better contactto other thin films and widens the latitude of subsequent processingsteps, since less rough films result in less translation of crests andvalleys in the film layers deposited thereover, less diffusereflectivity which makes photolithography easier, no need to clad thealuminum in the production of AMLCDs (rough aluminum traps charge whicheffects electronic performance [i.e., high or variable capacitance]),and more uniform etching.

The mechanical strength of the aluminum-containing films resulting fromthe process of the invention is higher than conventionally sputteredthin films of aluminum and some of its alloys. A high mechanicalstrength results in the resulting aluminum-containing films beingresistant to both electromigration and stress induced voiding.

This combination of such properties is superior to that of thin films ofaluminum and its alloys which are presently known. These properties makethe aluminum-containing films of the present invention desirable forelectronic device interconnects. These properties are also desirable inthin films for optics, electro-optics, protective coatings, andornamental applications.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming that which is regarded as the present invention,the advantages of this invention can be more readily ascertained fromthe following description of the invention when read in conjunction withthe accompanying drawings in which:

FIGS. 1 and 2 are illustrations of scanning electron micrographs of analuminum thin film produced by a prior art method before annealing andafter annealing, respectively;

FIGS. 3 and 4 are illustrations of scanning electron micrographs of analuminum thin film (Test Sample 1) produced by a method of the presentinvention before annealing and after annealing, respectively;

FIGS. 5 and 6 are illustrations of scanning electron micrographs of analuminum thin film (Test Sample 2) produced by a method of the presentinvention before annealing and after annealing, respectively;

FIG. 7 is an x-ray photoelectron spectroscopy graph showing the oxygencontent through the depth of an aluminum-containing film produced by amethod of the present invention;

FIG. 8 is a graph of roughness measurements (by atomic force microscopy)of various aluminum-containing films made in accordance with methods ofthe present invention; FIG. 9 is a cross-sectional side viewillustration of a thin film transistor utilizing a gate electrode andsource/drain electrodes formed from an aluminum-containing film producedby a method of the present invention;

FIG. 10 is a schematic of a standard active matrix liquid crystaldisplay layout utilizing column buses and row buses formed from analuminum-containing film produced by a method of the present invention;and

FIG. 11 is a cross-sectional side view illustration of interlevelshorting resulting from hillock formation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method of the present invention preferably involves using aconventional magnetron sputter deposition chamber within the followingprocess parameters:

    ______________________________________                                        Power (DC):      between about 1 and 4KW                                      Pressure:        between about 0.5 and 2.5 millitorr                          Argon Gas Flow Rate:                                                                           between about 25 and 90 sccm                                 Hydrogen Gas Flow Rate:                                                                        between about 50 and 400 sccm                                Oxygen Gas Flow Rate:                                                                          between about 0.25 and 2 sccm                                Argon:Hydrogen Gas Ratio:                                                                      between about 1:1 and 1:6                                    ______________________________________                                    

The operation of the magnetron sputter deposition chamber generallyinvolves applying the direct current power between the cathode (in thiscase the aluminum target) and the anode (substrate) to create theplasma. The chamber is maintained within the above pressure range and anappropriate mixture of argon gas, hydrogen gas, and oxygen gas isdelivered to the chamber. The aluminum-containing films resulting fromthis method have between about 12 and 30% (atomic) oxygen in the form ofaluminum oxide (Al₂ O₃) with the remainder being aluminum.

It is believed that the primary hillock prevention mechanism is thepresence of the hydrogen in the system, since it has been found thateven using the system with no oxygen or virtually no oxygen present(trace amount that are unmeasurable by present equipment and techniques)results in a hillock-free aluminum-containing film. It is also believedthat the presence of oxygen in the film is primarily responsible for asmooth (less rough) aluminum-containing film, since roughness generallydecreases with an increase in oxygen content in the film.

EXAMPLE 1

A control sample of an aluminum film coating on a semiconductorsubstrate was formed in a manner exemplary of prior art processes (i.e.,no hydrogen gas present) using a Kurdex-DC sputtering system to depositaluminum from an aluminum target onto a soda-lime glass substrate.

The substrate was loaded in a load lock chamber of the sputtering systemand evacuated to about 5×10⁻³ torr. The load lock was opened and a maindeposition chamber was evacuated to about 10⁻⁷ torr before the substratewas moved into the main deposition chamber for the sputtering process.The evacuation was throttled and specific gases were delivered into themain deposition chamber. In the control deposition, argon gas alone wasused for sputtering process. Once a predetermined amount of argon gasstabilized (about 5 minutes) in the main deposition chamber, about 2kilowatts of direct current power was applied between a cathode (in thiscase the aluminum target) and the anode (substrate) to create theplasma, as discussed above. The substrate was moved in front of theplasma from between about 8 and 10 minutes to form analuminum-containing film having a thickness of about 1800 angstroms.

Table 1 discloses the operating parameters of the sputtering equipmentand the characteristics of the aluminum film formed by this process.

                  TABLE 1                                                         ______________________________________                                                           Control Sample                                             ______________________________________                                        Sputtering Process Parameters                                                 Power (KW)           2                                                        Pressure (mtorr)     2.05                                                     Gas Flow (sccm)      Argon = 90                                               Characterization Parameters and Properties                                    Thickness (Å)    1800                                                     Stress (dyne/cm.sup.2) (compressive)                                                               -4.94 × 10.sup.8 (C)                               Roughness (Å)    1480 (unannealed)                                                             2040 (annealed)                                          Resistivity (μΩ-cm)                                                                       2.70                                                     Grain Size (Å)   1000-1200                                                Hillock Density      approx. 2 to 5 × 10.sup.9 /m.sup.2                 ______________________________________                                    

The measurements for the characterization parameters and properties weretaken as follows: thickness--Stylus Profilometer and scanning electronmicroscopy; stress--Tencor FLX using laser scanning; roughness--atomicforce microscopy; resistivity--two point probe; grain size--scanningelectron microscopy; and hillock density--scanning electron microscopy.

FIG. 1 is an illustration of a scanning electron micrograph of thesurface of the aluminum film produced under the process parametersbefore annealing. FIG. 2 is an illustration of a scanning electronmicrograph of the surface of the aluminum-containing film produced underthe process parameters after annealing. Both FIGS. 1 and 2 showsubstantial hillock formation both before and after annealing.

EXAMPLE 2

Two test samples (test sample 1 and test sample 2) of an aluminum filmcoating on a semiconductor substrate were fabricated using the method ofthe present invention. These two test samples were also formed using theKurdex-DC sputtering system with an aluminum target depositing on asoda-lime glass substrate.

The operating procedures of the sputtering system were essentially thesame as the control sample, as discussed above, with the exception thatthe gas content vented into the main deposition chamber included argon,hydrogen, and oxygen (wherein oxygen is preferably introduced in anatmospheric air stream). Additionally, the pressure in the maindeposition chamber during the deposition and thickness ofaluminum-containing film was varied from that control sample for each ofthe test samples.

Table 2 discloses the operating parameters of the sputtering equipmentand the characteristics of the two aluminum films formed by the processof the present invention.

                  TABLE 2                                                         ______________________________________                                                     Test Sample 1                                                                            Test Sample 2                                         ______________________________________                                        Sputtering Process Parameters                                                 Power (KW)     2            2                                                 Pressure (mtorr)                                                                             0.66         2.5                                               Gas Flow (sccm)                                                                              Argon = 25   Argon = 90                                                       Hydrogen = 50                                                                              Hydrogen = 200                                    Oxygen Flow (sccm)                                                                           about 0.25 to 0.5                                                                          about 0.25 to 0.5                                 Characterization Parameters                                                   and Properties                                                                Thickness (Å)                                                                            2000         1800                                              Stress (dyne/cm.sup.2)                                                                       4.93 × 10.sup.8 (T)*                                                                 -1.6 × 10.sup.8 (C)**                       Roughness (Å)                                                                            980 (unannealed)                                                                           640 (unannealed)                                                 520 (annealed)                                                                             410 (annealed)                                    Resistivity (μΩ-cm)                                                                 6.4          7.2                                               Grain Size (Å)                                                                           400-600      400-600                                           Film Oxygen Content                                                                          approx. max. 25%                                                                           approx. max. 20%                                  Hillock Density                                                                              no hillocks present                                                                        no hillocks present                               ______________________________________                                         *Tensile                                                                      **Compressive                                                            

FIG. 3 is an illustration of a scanning electron micrograph of thesurface of the Test Sample 1 before annealing. FIG. 4 is an illustrationof a scanning electron micrograph of the surface of the Test Sample 1after annealing. FIG. 5 is an illustration of a scanning electronmicrograph of the surface of the Test Sample 2 before annealing. FIG. 6is an illustration of a scanning electron micrograph of the surface ofthe Test Sample 2 after annealing. As it can be seen from FIGS. 3-6, nohillocks form on either sample whether annealed or not.

EXAMPLE 3

A number of aluminum-containing films were made at different ratios ofAr/H₂ and various system pressures were measured for oxygen contentwithin the films. The oxygen gas flow rate was held constant at about 2sccm and the power was held constant at 2 KW. The oxygen content wasmeasure by XPS (x-ray photoelectron spectroscopy). The results of themeasurements are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Sample Ar/H.sub.2          Pressure                                                                              Oxygen Content                             Number (sccm)   Ar/H.sub.2 (millitorr)                                                                           Range (atomic %)                           ______________________________________                                        1      90/400   0.225      2.50    12-25                                      2      90/300   0.300      2.40    15-30                                      3      50/200   0.250      1.50    15-25                                      4      25/50    0.500      0.60    25-30                                      5      90/50    1.800      2.10    15-25                                      ______________________________________                                    

An XPS depth profile for sample 4 (Ar/H₂ (sccm)=25/50, pressure=0.60) isillustrated in FIG. 7 which shows the oxygen content varying on averagebetween about 25 and 30% (atomic) through the depth of the film.

FIG. 8 illustrates the roughness of the five aluminum-containing filmsamples. As FIG. 8 illustrates, the higher the amount of hydrogen gasdelivered to the sputter deposition chamber (i.e., the lower the Ar/H₂ratio--x-axis), the smoother the aluminum-containing film (i.e., lowerroughness--y-axis).

FIG. 9 illustrates a thin film transistor 120 utilizing a gate electrodeand source/drain electrodes which may be formed from analuminum-containing film produced by a method of the present invention.The thin film transistor 120 comprises a substrate 122 having analuminum-containing gate electrode 124 thereon which may be produced bya method of the present invention. The aluminum-containing gateelectrode 124 is covered by an insulating layer 126. A channel 128 isformed on the insulating layer 126 over the aluminum-containing gateelectrode 124 with an etch stop 130 and contact 132 formed atop thechannel 128. An aluminum-containing source/drain electrode 134 which maybe produced by a method of the present invention is formed atop thecontact 132 and the insulating layer 126, and contacts a picture cellelectrode 136. The aluminum-containing source/drain electrode 134 iscovered and the picture cell electrode 136 is partially covered by apassivation layer 138.

FIG. 10 is a schematic of a standard active matrix liquid crystaldisplay layout 150 utilizing column buses 152 and row buses 154 formedfrom an aluminum-containing film produced by a method of the presentinvention. The column buses 152 and row buses 154 are in electricalcommunication with pixel areas 156 (known in the art) to form the activematrix liquid crystal display layout 150.

Having thus described in detail preferred embodiments of the presentinvention, it is to be understood that the invention defined by theappended claims is not to be limited by particular details set forth inthe above description, as many apparent variations are possible withoutdeparting from the spirit or scope thereof.

What is claimed is:
 1. A substantially hillock-free aluminum-containingfilm consisting essentially of aluminum having an oxygen content ofleast about 12% (atomic).
 2. The substantially hillock-freealuminum-containing film of claim 1, wherein said oxygen content in saidaluminum is between about 12% and 30% (atomic).
 3. A flat panel displayhaving at least one conductive component formed from substantiallyhillock-free aluminum-containing film, wherein said substantiallyhillock-free aluminum-contain film consists essentially of aluminumhaving an oxygen content of least about 12% (atomic).
 4. The flat paneldisplay of claim 3, wherein said oxygen content in said aluminum of saidsubstantially hillock-free aluminum-containing film is between about 12%and 30% (atomic).
 5. The flat panel display of claim 3, wherein saidsubstantially hillock-free aluminum-containing film is formed by amethod comprising sputter depositing aluminum metal on a substrate inthe presence of hydrogen gas and oxygen gas.